The Bleaching Chemistry of Chlorine Dioxide and. Hydrogen Peroxide in
Brightness Development and on Brightness Stability. 3. Results and Discussion.
4.
INFLUENCE OF FINAL BLEACHING STAGE ON ECF PULP BRIGHTNESS DEVELOPMENT, REFINABILITY AND PULP PROPERTIES Dr. V.R. (Perry) Parthasarathy* Canexus North America Ltd, Houston, Texas, USA and Dr. Jorge L. Colodette Federal University of Vicosa, Vicosa, MG, Brazil 3rd International Colloquium on Eucalyptus Pulp, Belo Horizonte, Brazil, March 4-7, 2007 *Now with Weyerhaeuser Company, USA.
ACKNOWLEDGEMENT The authors (VRP and JLC) acknowledge the financial support by Canexus NA and Canexus Quimica Ltda Brasil for this project. Acknowledgements are also due to Gary Kubera, Brian Bourgeois and Mike Christian of Canexus NA and Pericles Dos Santos and Eduardo Chow of Canexus Quimica Ltda Brasil.
The authors also wish to place in record their acknowledgement and appreciation to the laboratory staff at the Cellulose and Pulp Laboratory at the Department of Forest Engineering, Federal University of Vicosa, MG, Brazil for the excellent work - part of the data of that work is used in preparing this manuscript.
Objective
Yesterday, Goran Gellerestedt, Vuorinen et al., Sevastyanova, et al., and Litia et al., explained the fundamental aspects of brightness development, and the role of various wood components including HexA’s role in the brightness stability of pulps subjected to ECF and TCF bleaching under different bleaching sequence. In this paper, we have taken an approach to see what effect the final stages of bleaching has on brightness development and brightness stability, refinability and properties.
Objective
The process used in the final stages of bleaching is important for reaching high brightness in pulps. High brightness and optically balanced pulp (as to their whiteness (L*) and a* and b* values) is imperative to produce ultra high brightness (>92 %ISO) and high whiteness (>100%) paper products with the use of as little Optical Brightening Agents (OBA) as possible. The two distinct final bleaching stages, DD and DP are compared with respect to their ability to produce not only very bright and optically balanced pulps but also pulps with low brightness reversion.
Approach
The paper is divided into four sections….. 1. 2.
3. 4.
“Color” Chemistry and Metrics Used to Track Optical Changes in Pulp The Bleaching Chemistry of Chlorine Dioxide and Hydrogen Peroxide in Brightness Development and on Brightness Stability Results and Discussion Conclusions with Recommendations
Brief Review on “Color” Chemistry Brightness is a measurement of pulp’s ability to reflect light in a very narrow blue band (457 nm )(Popson and Malthouse 1990) and in bleached pulps, it is a function of lightness or whiteness (L*), because the chromacity of these substrates are rather low. Chromacity is defined by CIELAB’s X-Y coordinates (in Hunter Space, it is a* (green-red) and b* (blue-yellow) values that defines the color space (Popson and Malthouse 1990)).
On the other hand, whiteness is a measurement of a pulp’s ability to reflect light across the entire color spectrum, color is defined by value, hue, and chroma (Judd and Wyscezi 1975).
CIE Chromacity Diagram
a*
b*
Brief Review on “Color” Chemistry For white and near-white papers, brightness is a single optical measurement. However, when pulps are bleached to very high brightness, a* (green-red) and b* (blueyellow) numbers do change. They have to be tracked so as to understand what influence they will have on the “Chromacity Value”.
The a* and b* values have to be optically balanced to minimize the “Grayness” of the pulp or sheet. In this work, chromacity values were used to track the efficiency of ECF bleaching and to measure the brightness reversion of DD and DP bleached pulps under different humidity conditions.
Brief Review on “Color” Chemistry TAPPI’s TAPPI T 524 om-94 sets the criteria for white or near-white paper, having L*≥ 84.0 and a chromacity value of (√ (a*2 + b*2) ² ≤ 10.0. This specification of L*a*b* is not adequate to define pulps bleached to very high brightness.
A unified measurement called “Chromacity” defines the changes in color on the Hunter’s color space. An extension to the “Chromacity” measurement is “Color Index (CI)” that takes into account the difference in the lightness, darkness, and the chroma of the sample and the standard.
Brief Review on “Color” Chemistry
Color Index =√ (∆L*2+∆a*2+∆b*2) where ∆L* = Delta of whiteness before and after aging ∆a* = Delta of a* before and after aging ∆b* = Delta of a* before and after aging
Brief Review of Bleaching Chemistry Chlorine dioxide is a very specific oxidant. It prefers to react with residual lignin instead of cellulose fractions of pulp.
Certain decomposition and disproportionation products of ClO2 give rise to a number of oxidative species, including radical species like ClO. and Cl.. These are indiscriminate oxidants and attack both lignin and carbohydrate structures in pulp. The attack on lignin by these radicals does result in the generation of quinones whereas the attack on carbohydrates does not result in the oxidation of C2 or C3 carbon in the cellulose molecule to the formation of carbonyl groups but the oxidation of C6 carbon to glucouronic acid (Stewart and Smelstorius 1968 and Miyazaki 1971).
Brief Review of Bleaching Chemistry
The decomposition products of H2O2 - the hydroxyl and hydro peroxyl anions react mostly with lignin structures but the hydroxyl radicals, a strong and indiscriminate oxidant (electrophile) reacts with both residual lignin as well as carbohydrates. Reactions of hydroxyl radicals with the carbohydrates result in the oxidation of C2 and C3 carbon positions in the cellulose and hemicelluloses structures to generate carbonyl groups.
The excessive presence of carbonyl groups in cellulose structures (Andrady and Parthasarathy (1991) is attributed for the thermal yellowing (brightness reversion) of hydrogen peroxide bleached pulps of high lignin content.
Brief Review of Bleaching Chemistry In dye chemistry, the fastness of color using a particular dye is due to the extent of changes in quinone (color) structures to quinhydroxy (colorless) structures (Finar 1956). Lachanel and Chirat (2004 and 2005) showed that the residual lignin in unbleached pulps differed in their quinone concentration and that the pulps which were more difficult to bleach were more colored and richer in quinone structures, with a more pronounced reddish hue (a* (greenred coordinates) in the L*a*b* space).
It is theorized by Lachanel and Chirat (2005) that since quinones are not easily degraded by ClO2, pulp bleachability would be related to the amount of quinone groups and by proxy by tracking the value of a* in the L*a*b* space.
Brief Review of Bleaching Chemistry
Brightness reversion of pulp is a complex phenomenon having initiated by light and thermal radiation and a combination of both. Also, the extent of brightness reversion depends not only on the intensity of the light or heat but also on the ambient moisture conditions (Relative Humidity)
Market Pulps shipped to different destinations are subject to cyclical changes of both heat and relative humidity resulting in changes in the brightness at delivery points. It is therefore important for market pulps that not only the measurement of final brightness but also the brightness stability is critical.
Brief Review of Bleaching Chemistry The objective is to verify the assertion by Lachanel and Chirat (2005) that peroxide is better in reducing the quinone (color) structures in pulps to quinhydroxy (colorless) structures than chlorine dioxide which then implies that bleach plants with final P-stage are better suited to produce high brightness pulps than D-stage.
To compare the efficiency of a bleaching process, particularly in providing very high brightness and brightness stability in Eucalyptus market pulps, it is important to have a unified measurement. In this study we have used the chromacity measurements to track the efficiency of bleaching of pulps to very high brightness.
Properties of Oxygen Delignified Pulp
Oxygen Delignified Eucalyptus Kraft Pulp Brightness, % ISO
47.6
Kappa Number
12.5
Viscosity, cP
31.0
HexA, mmol/kg
69.5
COD, kg O2/t
7.6
Chlorine Dioxide Delignification Conditions Two Different Initial Chlorine Dioxide Delignification of Oxygen Delignified Pulps were employed. (1) Conventional Chlorine Dioxide (Do) and (2) Hot Chlorine Dioxide Delignification (DHT). The Delignification Conditions are Detailed Below:
Temperature, oC Time, minutes
60, 90, 95 15, 30, 60, 90, 120 H2SO4
Acid Used End pH at the D-Stage
2.50
3.0
ClO2, as % Active Cl2
2.55
2.55
H2O2, % NaOH, %
0.50 1.20
0.50 1.20
Acid Charge, %
0.30
0.40
Total Active Cl2, %
3.60
3.60
Bleaching Conditions Chlorine Dioxide delignified (Do and DHT) pulps were bleached by E(P)DD, E(P)DP and E(P)DDP processes. Detailed bleaching conditions are listed elsewhere*
“Final D-stage versus Final P-stage for high brightness ECF bleaching of eucalyptus Kraft pulps, which is the best?”. Submitted to Canexus Limited 450 Gears Road, Suite 400 , Houston Texas USA. Report # 027/2006 by Jorge Luiz Colodette, Rubens Chaves de Oliveira, Marcos Antônio Bandeira Azevedo, Oldair Cristino de Paula, Luciano Sabioni, and Carla L. Lopes Motta
Brightness Reversion Measurements Brightness reversion was tested under two different aging conditions: (1) Wet (60ºC, 7 days, 90% RH) (Finnish Pulp and Paper Institute) (Modified). Finnish Pulp and Paper Institute’s (KCL) suggested method for the wet reversion test (Modified). For the wet reversion test, the original KCL procedure calls for subjecting the pulp sheets at 100% RH at 100oC for one hour. In this study, the procedure was modified and the exposure time prolonged to 7 days at lower humidity levels (90% RH) and lower temperature (60oC) to simulate the conditions that the market pulps would be subjected to when shipped overseas.
(2) Dry (105ºC, 4 hours, 0% RH) (TAPPI Useful Method)
RESULTS AND DISCUSSION
Relationship Between Brightness and Whiteness of Eucalyptus Pulps Brightness Versus Whiteness 98.10 y = 0.2235x + 77.697 R2 = 0.8725
Whiteness
98.05
98.00
97.95
97.90
97.85 90.20
90.40
90.60
90.80
Brightness, %ISO
91.00
91.20
Brightness and Whiteness of DD and DP Bleached Eucalyptus Pulps (Do Delignification) Conventional Chlorine Dioxide Delignification (DO) and Multi-stage Bleaching Bleaching Sequence DOE(P)D1P
Final Stage Temperature o C 70.0
90.0
DOE(P)D1D2
70.0
90.0
End pH
9.0 10.0 11.0 9.0 10.0 11.0 4.5 6.0 9.0 4.5 6.0 9.0
Brightness % Lightness or ISO Whiteness (L*) 90.60 98.00 90.40 97.92 90.50 97.89 90.50 97.94 90.00 97.80 90.60 97.93 90.50 97.91 90.37 97.88 90.60 97.93 90.70 98.08 90.30 97.85 90.53 97.95 91.50 98.13 91.10 98.11 90.80 98.01 91.13 98.08
Average
Average
Average
Average
Brightness and Whiteness of DD and DP Bleached Eucalyptus Pulps (DHT Delignification) Hot Chlorine Dioxide Delignification (DHT) and Multi-stage Bleaching Bleaching Sequence DHTE(P)D1P
Final Stage Temperature o C 70.0
90.0
DHTE(P)D1D2
70.0
90.0
End pH
9.0 10.0 11.0 9.0 10.0 11.0 4.5 6.0 9.0 4.5 6.0 9.0
Brightness % Lightness or ISO Whiteness (L*) 91.00 97.97 90.90 97.95 90.90 97.95 90.93 97.96 91.00 98.06 91.10 98.03 91.30 98.08 91.13 98.06 91.50 98.13 91.20 98.14 90.70 97.95 91.13 98.07 91.50 98.13 91.10 98.11 90.80 98.01 91.13 98.08
Average
Average
Average
Average
Brightness Reversion (PC Numbers) For DD and DP Bleached Pulps Hot Chlorine Dioxide Delignification (DHT) and Multi-stage Bleaching Bleaching Sequence
DHTE(P)D1P
Final Stage Temperature oC
70.0 90.0
Average DHTE(P)D1D2
70.0 90.0
Average
Brightness % ISO
90.93 91.13
Brightness Reversion Wet Dry Method* Method** 88.87 89.17 89.13 89.63
Post-Color Number Wet Method* 0.212 0.208
Dry Method** 0.196 0.181
91.03
89.00
89.40
0.210
0.188
91.13 91.13
88.23 88.23
88.10 87.97
0.250 0.250
0.256 0.261
91.13
88.23
88.03
0.250
0.259
Conventional Chlorine Dioxide Delignification (DO) and Multi-stage Bleaching Bleaching Sequence
DoE(P)D1P
Final Stage Temperature oC
Brightness % ISO
70.0 90.0
90.50 90.37
Average DoE(P)D1D2
Average
70.0 90.0
Brightness Brightness Reversion Reversion Wet Dry Wet Dry Method* Method** Method* Method** 87.90 88.40 0.238 0.214 87.87 88.10 0.234 0.223
90.43
87.88
88.25
0.236
0.218
90.53 91.13
86.43 88.23
87.90 87.97
0.298 0.250
0.239 0.261
90.83
87.33
87.93
0.274
0.250
*Wet Reversion Test: 60ºC, 7 Days, 90% Relative Humidity **Dry Reversion Test: 105ºC, 4 Hours, 0% Relative Humidity
Summary of Color Index (CI) Values for DD and DP Bleached Pulps Hot Chlorine Dioxide Delignification (DHT) and Multi-stage Bleaching Bleaching Sequence
Whiteness, % (L*)
Chroma Values a*
Color-Index
Average
Delta
1
(between DP and DD bleaching)
b*
Wet Dry Wet Dry Wet Dry Wet Dry Method* Method** Method* Method** Method* Method** Method* Method** DHTE(P)D1P
Before Aging 98.01 98.01 -0.10 -0.10 2.78 After Aging 97.40 97.71 -0.10 -0.10 3.17 DHTE(P)D1D2 Before Aging 98.08 98.08 -0.08 -0.08 2.86 After Aging 97.19 97.29 0.28 -0.02 3.43 Conventional Chlorine Dioxide Delignification (DO) and Multi-stage Bleaching Bleaching Sequence
Whiteness, % (L*)
DoE(P)D1P DoE(P)D1D2
2.78 3.50 2.86 3.80
Chroma Values a*
0.72
0.78
0.75
1.12
1.23
1.17
Color-Index
Average
*Wet Reversion Test: 60ºC, 7 Days, 90% Relative Humidity **Dry Reversion Test: 105ºC, 4 Hours, 0% Relative Humidity 1. Delta is calculated between DP and DD bleaching between likely testing events (before aging and after aging events)
Delta
1
(between DP and DD bleaching)
b*
Wet Dry Wet Dry Wet Dry Wet Dry Method* Method** Method* Method** Method* Method** Method* Method** Before Aging 97.9 97.91 -0.09 -0.09 3.07 3.07 After Aging 97.1 97.35 0.21 -0.02 3.60 3.83 0.99 0.95 Before Aging 98.0 98.02 -0.10 -0.10 3.00 3.00 After Aging 96.9 97.20 0.33 0.09 3.65 3.86 1.33 1.20
-0.42
0.97 1.27
-0.30
Properties of Bleached Pulps from DHTE(P)D1D2P and DHTE(P)D1P Processes.
Bleaching
Brightne
Sequence
ss, %ISO
Whiteness, %
Chroma Values
(L*)
D E(P)D D P HT 1 2
D E(P)D P HT 1
a*
Wet
Dry
Wet
Method*
Method**
Method*
Color-Index
Average
b* Dry
Wet
Method** Method*
Dry
Wet
Dry
Method**
Method*
Method**
0.74
0.77
0.76
0.73
0.78
0.75
Before Aging
92.0
98.31
98.41
-0.11
-0.09
2.74
2.54
After Aging
89.7
97.59
98.01
-0.09
-0.10
2.93
3.20
Before Aging
91.0
98.01
98.01
-0.10
-0.10
2.78
2.78
After Aging
89.2
97.40
97.71
-0.10
-0.10
3.18
3.50
CONCLUSIONS Final brightness of HW pulps, in particular, eucalyptus pulps is dictated by the type of delignification employed prior to bleaching. Hot chlorine dioxide delignification (DHT) resulted in higher brightness pulps over conventional Do pulps.
At a given OXE, DD can give a higher brightness pulps over DP with statistically insignificant changes in reversion, CI and chromacity values. Adding a peroxide polishing step to the DD bleaching will lead to not only high brightness pulps but a pulp optically balanced in its a* and b* values and that the papermakers need to use less OBA to reach “pure” white paper (>100% Whiteness) It is estimated that between DD and DP bleaching, the former can bleach pulps to >92 %ISO brightness at US$ 2 to US$ 3 per to less than the latter, a significant cost advantage of having a final D-stage.
CONCLUSIONS Refining the pulps bleached to high end pH is easier than pulps bleached to low end pH.
Between DD and DP bleached pulps, the former provided a better tensile and tear index at a given CSF than the latter but the difference is statistically insignificant (Table 12).
OBrigado
